1. Field of the Invention
This invention relates to the production of sulfur and is particularly concerned with forming sulfur by reacting hydrogen sulfide with sulfur dioxide in the presence of liquid organic disulfides.
2. Description of the Prior Art
Hydrogen sulfide is a noxious gas which is frequently found in natural gas streams, in refinery and chemical plant process streams and in waste gas streams vented to the atmosphere from industrial sources. The hydrogen sulfide is normally removed from these streams in order to decrease corrosion in processing equipment, prevent fouling of catalysts and to comply with environmental emission standards. For economic reasons the recovered hydrogen sulfide is normally converted to elemental sulfur. The process most widely used for converting hydrogen sulfide to elemental sulfur is the Claus process.
In the Claus process a stream of hydrogen sulfide is fed into a reaction furnace where it is burned with sufficient air to convert one third of the hydrogen sulfide to sulfur dioxide. The hold-up time in the reaction furnace is such that the sulfur dioxide generated will partially react with the hydrogen sulfide to form sulfur vapor. The reaction takes place at about 2000.degree. F. with no catalyst present. Approximately a 70% overall conversion of hydrogen sulfide to sulfur can be realized in this step. Next, the hot reaction gases are passed through a waste heat boiler and into a sulfur condenser where the sulfur produced in the thermal conversion step is removed. The gases are then reheated and injected into a catalytic converter where they are passed through a bed of catalyst, normally bauxite, to obtain further conversion of hydrogen sulfide to sulfur. The product gases are then passed through a condenser for removal of sulfur formed in the first converter. The gases from the condenser are then reheated and passed through a second catalytic converter to produce further sulfur. As many catalytic converters as desired can be used, however, experience has shown that two or three catalytic converters is usually the optimum choice.
A Claus conversion unit will generally yield about a 95% conversion of the hydrogen sulfide fed to the unit. This means that the tail gas from a Claus unit will normally contain sufficient residual sulfur compounds to violate emission control standards if it is vented to the atmosphere. It is therefore normally necessary to integrate a tail gas cleanup process with the Claus conversion plant.
The Claus process for converting hydrogen sulfide into sulfur has pronounced disadvantages. It may require a high capital investment, is expensive to operate, utilizes high temperatures, and results in only about a 95% conversion of hydrogen sulfide. The level of conversion obtained in a Claus unit may necessitate the addition of a tail gas treating process in order to meet emission regulations. Such a process may be complex and add substantially to the investment and operating costs of the sulfur recovery unit.
Several processes have been proposed to alleviate the problems associated with the Claus process. These processes normally consist of passing hydrogen sulfide and sulfur dioxide or gases containing these substances into a liquid organic solvent which preferentially absorbs the hydrogen sulfide and sulfur dioxide and serves as a medium for their reaction to form elemental sulfur. These processes differ somewhat in the manner and order in which the hydrogen sulfide and sulfur dioxide are passed into the solvent reaction medium, but the main factor that distinguishes these liquid phase processes from one another is the type of organic solvent that is used. For example, one process utilizes an aqueous solution of a glycol such as diethylene glycol, triethylene glycol, propylene glycol or the like as the solvent. Another process employs a carboxamide such as dimethylformamide, diethylformamide, diethylacetamide, or the like; while yet another process uses a sulfone such as sulfolane as the solvent.
Although the liquid phase solvent processes referred to above and similar processes for the production of sulfur have advantages over the Claus process in that they are simple, operate at low temperatures and result in higher conversions of hydrogen sulfide to elemental sulfur, they also have some pronounced disadvantages. The solvent utilized to absorb the reactant gases may be expensive, may not have a high absorptive capacity for both hydrogen sulfide and sulfur dioxide and may yield a flocculent or tacky sulfur which is difficult to recover by commercial methods of separation.